Handles validator disabling in the backing subsystem. The PR introduces
two changes:
1. Don't import statements from disabled validators.
2. Don't validate and second if the local validator is disabled.

The purpose of this change is first to ignore statements from disabled
validators on the node side. This is an optimisation as these statements
are supposed to be cleared in the runtime too (still not implemented at
the moment). Additionally if the local node is a validator which is
disabled - don't process any new candidates.

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Co-authored-by: ordian <noreply@reusable.software>
Co-authored-by: ordian <write@reusable.software>

Co-authored-by: Tsvetomir Dimitrov <tsvetomir@parity.io>